1. Field of the Invention
This invention relates to automatic positioning of a mechanical arm, and in particular, to such automatic positioning with a nonlinear drive mechanism.
2. Description of the Related Art
In the burgeoning field of robotics, it has become commonplace to control the movement of a robot arm automatically, for example. Such an arm may be controlled in both angular and linear displacement with numerous available degrees of freedom. For example, an arm may be rotated about fixed axes fixed in three different coordinates with those axes which are orthogonal to the axis of the arm being located at different pivot points along the arm or at one end thereof. Linear positioning of the arm may also be effected, and control of a combination of these types of movement may be arranged. Numerous approaches to the controlling or positioning of a mechanical arm may be found in the prior art, as exemplified by the following U.S. patents and publication.
U.S. Pat. No. 3,261,967 to Rosin et al. discloses a mechanical device for measuring a dimension of any object which can be viewed with a TV camera. The TV camera is placed to view a point of reference on the object being measured and is then made to move and scan the object in the dimension to be measured. The amount of camera movement is proportional to the distance scanned on the object and thus a dimension may be measured by measuring the amount of movement. The mechanical device controlling the movement of the camera is nonlinear, and calculations are required to translate the mechanical movement into meaningful data.
The following U.S. patents disclose control systems for stepping motors: U.S. Pat. Nos. 3,324,369 to Markakis, 3,345,547 to Dunne, 3,353,076 to Haines, 3,426,258 to Van Pelt, and 3,573,593 to Beery. In all these patents the control system disclosed makes use of photocells and digital logic functions. Except for the hydraulic control mechanism disclosed in Van Pelt, all the control systems disclosed are electrical.
U.S. Pat. No. 3,660,746 to Milek discloses a system to prevent oscillation in a stepper motor as it advances from one position to another. Electronic damping is provided by digital circuits which advance a rotor from position to position while controlling the energization of all the stator windings.
U.S. Pat. No. 3,683,254 discloses a servo system arranged to reduce the effect of noise on system performance.
A drive circuit for providing high-level driving current to the stator coils of a stepping motor during stepping operation is disclosed in U.S. Pat. No. 3,893,012 to Lin.
In U.S. Pat. No. 3,748,566 a method of compensation for the angular error of an electric stepping motor is disclosed. An electrical error signal is generated whenever the rotor of the stepping motor deviates from a selected stepping position. This signal is used to correct or adjust the individual winding currents being supplied to the motor.
U.S. Pat. No. 3,719,878 to Ferguson et al. discloses an automatic control system using digital techniques to drive a stepper motor. A preselected output position triggers a synchronizing signal to insure periodic alignment between the input signal and the corresponding output control position. A monitoring capability is provided by a memory comparison system.
U.S. Pat. No. 3,942,619 to Nordstrom et al. discloses a printing apparatus including a multi-phase stepping motor with an opto-electronic control system. The stepper motor has photodetectors coupled to the shaft to detect mechanical position and movement. The system disclosed is for controlling linear mechanical movement with no attempt made to select specific mechanical positions.
U.S. Pat. No. 3,961,232 to Newell discloses a hybrid step motor system that utilizes coarse and fine stepper motors. The linear digital system disclosed requires binary coded decimal data as an input.
In U.S. Pat. No. 4,039,919 to Suzaki et al., a multi-phase motor controller is disclosed with a photodetector on the motor shaft to sense and control motor rpm.
An incrementally controllable motor drive system is disclosed in U.S. Pat. No. 4,042,863 to Von der Heide. Photodetectors and digital logic circuits are used to control the execution of steps having a preselected step length.
A control system for a stepper motor which utilizes velocity squared feedback is disclosed in U.S. Pat. No. 4,042,868 to Rhodes.
U.S. Pat. No. 4,107,595 to Campe discloses a circuit for controlling the power or current applied to a stepping motor in order to control the speed of the motor.
U.S. Pat. No. 4,158,800 to Jahelka et al. discloses digital circuitry for controlling the speed of a stepper motor. A closed loop feedback system is used which includes a coded disk driven by the motor and decoding means to derive signals from the disk.
U.S. Pat. No. 4,234,787 to Hutter et al. discloses an apparatus for monitoring the motion of any device which is caused by a turning wheel or which causes a wheel to turn. The motion is measured by counting pulses which are developed as the wheel is turned. A reference pulse is developed by some other actuating device at a specific place in the mechanism movement. The counter output is proportional to the movement of the mechanism from the reference point.
U.S. Pat. No. 4,110,828 to Baumgartner et al. discloses an error-preventing incremental measuring system utilizing a plurality of scanning elements associated to a relatively movable divided scale. The system combines the signals from the scanning elements by way of a logical network.
U.S. Pat. No. 4,289,983 to Bengnar et al. discloses an encoder for monitoring bidirectional motion. A disk with holes in it is attached to the armature of a stepper motor and the disk is allowed to rotate between a light emitter and a photodetector. A novel hole shape allows a single detector-emitter pair to be used for both clockwise and counterclockwise rotation.
U.S. Pat. No. 3,811,648 to Ream, Jr. et al. discloses a system for remotely positioning and displaying the position in azimuth of a movable instrumentation package. The package is mounted on a carriage driven by an electric motor around a circular track. A light beam directed toward two photocells is broken by a notched metal strip as the carriage moves along the track. The pulses generated by the photocells are counted, and the counter is reset every ten degrees by mechanical switches to cancel any small accumulated azimuth error.
In Vol. 19, No. 4, of IBM Technical Disclosure Bulletin of September, 1976, Fleek et al. disclose a sequence control for stepper motors. The circuit arrangement disclosed provides a control for operating a three-phase stepper motor in either full- or half-step mode. It also provides control for single-phase operation in a full-step mode.
None of the references described above disclose a solution to the problem of providing automatic positioning control with a nonlinear drive mechanism to provide linear angular movement. If a nonlinear drive mechanism is used, a given number of revolutions of the drive motor at one point will not produce the same angular displacement as the same amount of drive revolutions at another point.